In a known electronic musical instrument, a melody reproduction apparatus, or the like which is designed to generate musical tones, an envelope waveform is applied to the waveform data read out from a waveform ROM having particular musical-tone waveforms stored therein, to generate a musical tone. Examples of such an envelope waveform application circuit include one in which CR discharge characteristics are utilized, one which is equipped with a ROM stored therein with PCM data for an envelope waveform, and the like.
The first-mentioned envelope waveform application circuit, as shown in FIG. 12(a), comprises a transistor Tr, a capacitor C, and a resistor R. A pulse shown in FIG. 12(a) by 12aA is applied to a gate of the transistor Tr to open the transistor Tr. The capacitor C is thereby electrically charged, and the electric charge thus obtained is discharged via the resistor R. Through this charging and discharging operation, an envelope waveform shown in FIG. 12(a) by 12aB is obtained. This envelope waveform 12aB constitutes an envelope of the output signal from a D/A converter 12a2 for providing D/A conversion of the waveform data 12aC read out from a waveform ROM 12a1.
The second-mentioned envelope waveform application circuit includes an envelope ROM 12b1 having stored therein, in the form of PCM data, the envelope waveform shown in FIG. 12(b) by 12bA, for example, which includes sound-volume level data varying with time. The sound-volume level data is read out from that envelope ROM 12b1 and is multiplied by the waveform data 12bB read out from a waveform ROM 12b2, in a multiplication circuit 12b3. The resulting product is subjected to D/A conversion by a D/A converter 12b4 to obtain a musical tone.
However, in the first-mentioned envelope waveform application circuit only a simple envelope waveform can be obtained, posing the problem that it is not easy to modify the waveform. Further, as the number of musical tones to be simultaneously generated increases, the respective number of transistors, capacitors and resistors also increases, which poses a problem that the density of the circuit is increased and the cost is also increased.
Further, the second-mentioned envelope waveform application circuit is required to have a large storage capacity (several K bits to several tens of K bits) and therefore is difficult to integrate.